1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor article and a semiconductor article, particularly for high-frequency applications.
2. Description of the Background Art
DE 43 01 333 A1, which corresponds to U.S. Pat. No. 5,424,227, discloses different process steps in a manufacturing process for an NPN-silicon-germanium heterobipolar transistor with a P-silicon-germanium base layer. Reference is made to the entire scope of this publication and the process steps in FIGS. 1a to 1h are explained.
FIG. 1a: On the surface of a P-doped silicon substrate wafer 1s, a (masking) oxide layer 10s is formed by thermal oxidation and patterned with photoresist; an N+ collector connection layer 2s (“buried layer”) is formed by implantation and post-diffusion of arsenic ions.
FIG. 1b: After removal of the masking oxide layer 10s, the layer sequence of the SiGe HBT is grown in a single process (for example, using molecular beam epitaxy MBE) without interruption, with simultaneous doping of the layers. The layer sequence comprises the N−-collector layer 3s (for example, with a layer thickness of 300 nm and a doping concentration of 1016 to 1017 cm−3), the P+ SiGe base layer 4s (germanium proportion of, for example, 20%, layer thickness of 50 nm, and doping concentration of 1019 cm−3), the N−-emitter layer 5s (for example, with a layer thickness of 100 nm and a doping concentration of 1018 cm−3), and the N+ emitter contact layer 6s (for example, layer thickness of 50 nm and doping concentration exceeding 1020 cm−3).
FIG. 1c: An auxiliary layer 11s (for example, of silicon nitride Si3N4) is deposited over the full surface of the N+ emitter contact layer 6s (for example, by a CVD method), with a layer thickness of approximately 0.3 μm and patterned with photoresist; part of the Si3N4 auxiliary layer 11s and the Si emitter contact layer 6s is etched off anisotropically—the emitter layer 5s continues to remain above the SiGe base layer 4s. 
FIG. 1d: A CVD oxide (TEOS) 12s (for example, having a layer thickness of 0.3 μm) is conformally deposited over the surface of the semiconductor array and etched back anisotropically in such a way that an oxide layer 12s (spacer) remains only at the vertical edges of the auxiliary layer 11s and of the emitter contact layer 6s, and the remaining surface (for the base connection) is exposed; BF2 ions (dose, for example, 4×1015 cm−2, acceleration voltage, for example, 40-80 kV) are implanted in the exposed surface to a depth just below the SiGe base layer 4s and hence the emitter region 5s at the side of the oxide spacer 12s is redoped to provide the P+ base connection region 7s, and the base collector PN junction 9s is moved to underneath the base/collector boundary layer 9as, and the emitter/base/PN junction 8s to underneath the spacer oxide layer 12s. 
FIG. 1e: The final mesa-type structure is formed by the base connection region 7s and the collector layer 3s is patterned by anisotropic etching; by deposition and patterning of a CVD oxide (layer thickness, for example, 0.5 μm), field oxide regions 14s are formed that separate the various elements or active structures of the integrated circuit from one another.
FIG. 1f: By thermal oxidation at a temperature of, for example, 700° C., the exposed surface of the semiconductor array is covered with an SiO2 layer 13s with a thickness of, for example, 10 nm. Both the base/emitter/PN junction 8s and the collector/base/PN junction 9s are passivated on the surface and hence protected from leakage currents.
FIG. 1g: A CVD oxide 15s (TEOS) is deposited over the entire surface of the semiconductor array (for example, with a layer thickness of 0.5 μm) and etched back anisotropically in such a way that, on the one hand, the collector contact opening 20s is formed and, on the other, oxide spacers 15s remain, which protect the surface of the base/emitter/PN junction 8s over a length d2. The distances d1 (from the edge of the emitter contact layer 6s to the base connection region 7s) and d2 (from the base connection region 7s to the later base metal layer 16s) are self-aligned and minimal thanks to the spacer technique described and use of the oxide layers 12s and 15s. 
FIG. 1h: The remainder of the Si3N4 auxiliary layer 11s is removed, and the emitter contact opening 21s is exposed. By vapor deposition of a metal layer 16s (for example, titanium) and temperature treatment of said layer, a metal silicide (for example, titanium silicide) is formed on the surface of the silicon areas 2s, 6s, 7s, whereas the pure metal remains on the surface of the oxide areas 13s, 14s, 15s; the metal layer above the oxide areas 13s, 14s, 15s is selectively removed using an etchant solution. It is then possible, after application of an oxide layer and the manufacture of contact openings for emitter, base, and collector, to apply a metallized layer using conventional methods. SiGe HBTs of this type of FIG. 1h, however, can be produced not only on simply patterned substrates as described, but also, for example, on substrates already containing finished components in pure silicon technology (e.g., MOS or bipolar).
The base of the previously described transistor can be conceptually subdivided into an intrinsic base and an extrinsic base. The intrinsic base with the adjacent emitter and collector regions forms level and substantially parallel pn junctions, whereas the extrinsic base is made to provide the electrical connection of the intrinsic base to at least one external contact.
For many applications of bipolar transistors, thus, for example, for high-frequency power amplification, the base resistance is an important parameter, which limits the electrical properties of the transistors, particularly their cutoff frequency. Frequently, the base resistance is dominated by the resistance of the extrinsic base.
To reduce the resistance of the extrinsic base, the extrinsic base in a high-doped semiconductor material is made of the same type of dopant as the intrinsic base, but with a much greater layer thickness, in order to achieve a low layer resistance in this way. In the method taught by the publication “IEEE-IEDM 2002, Article 31.6.1,” for example, the extrinsic base is made of polysilicon with a layer thickness that is far greater than the width of the intrinsic base. Moreover, the extrinsic base can be made as a high-doped implanted region.
In order to reduce the resistance of the extrinsic base of bipolar transistors in silicon technology, the extrinsic base is silicidized, as set forth, for example, in the publications U.S. Pat. Nos. 5,459,084, 6,177,717, US 2002/0168829, or U.S. Pat. No. 6,465,317. Here, a layer of a metal capable of reacting with silicon is applied to the extrinsic base and the layer is caused to react with the silicon material of the extrinsic base. By annealing above a transformation point, the forming suicides are converted to a modification with a low specific resistance.
It is desirable for many applications, particularly in the high-frequency range, to introduce another semiconductor material, for example, germanium or carbon, in addition to silicon, into the intrinsic base of a bipolar transistor to manufacture a heterobipolar transistor. During the epitaxial deposition of the semiconductor material for the intrinsic base, the additional semiconductor material is also introduced into parts of the extrinsic base in addition to silicon. Germanium and carbon behave chemically like silicon, so that a metal capable of reacting with silicon also reacts with germanium or carbon under the same reaction conditions.
Particularly compounds of titanium and germanium exhibit a lower thermal stability than homologous compounds of titanium and silicon and dissociate at the transition point of the silicides. The germanium and titanium separation products formed during the dissociation act to increase the resistance.
In the presence of another semiconductor material (germanium) in the intrinsic base, it can therefore be desirable to carry out a silicidation reaction in such a way that the silicidation front does not penetrate deep into the intrinsic base. Nevertheless, a high silicide thickness is desirable to reduce base resistance.
As described previously in DE 43 01 333 A1, a thick emitter layer 5s is provided for this purpose, which has the function of keeping the silicon material available for a thick silicide layer 16s, without the silicidation front penetrating deep into the layer 4s of the silicon-germanium composition. A pn junction with a relatively large area is formed at the edge of the thick emitter area, the pn junction lying completely within the silicon. A relatively large silicon diode is connected in particular to the base/emitter diode of the heterobipolar transistor. This influences the electrical properties of the forming element and limits its geometric scaleability.
DE 198 45 790 A1 discloses a manufacturing process for SiGe heterobipolar transistors without large silicon edge transistors, in that for the wet-chemical thinning of silicon layers in the active emitter region of a bipolar transistor, additional doping, having a thickness less than 3 nm, introduced by means of “atomic layer doping” (ALD) in a cover layer, acts as an etch stop layer for wet-chemical etchant. The etch stop layer is subsequently removed with a wet-chemical etchant.
The layer growth of the cover layer occurs further above the etch stop layer as single crystals, so that with the aid of a silicon etchant, the single-crystal silicon of the cover layer can be removed highly selectively to the etch stop layer and to a dielectric. In an embodiment of the method of DE 198 45 790 A1, the epitaxial deposition of the thick emitter layer is interrupted by the application of the thin etch stop layer, for example, consisting of high-p-doped silicon, which acts as an etch stop, at the site of the emitter window during selective, wet-chemical thinning of the thick emitter layer for deepening the emitter window.
The publication “IEEE IEDM 2003, Technical Digest, Article 5.3.1” also describes a manufacturing method with relatively large edge transistors in which a high silicide thickness is achieved on the extrinsic base, whereas simultaneously the silicidation front above the germanium-containing layer is brought to a standstill. To that end, after the completion of the emitter contact, the silicon material is deposited exclusively above the extrinsic base by selective epitaxy and then silicidized.
A further disadvantage of the prior art arises from the fact that a silicide-silicon interface, forming during the silicidation, is generally formed unevenly. There is the particular risk of the formation of needle-shaped or pyramidal silicide crystallites, which project into the silicon area. To avoid a short circuit (“spike”) between the silicide layer on the extrinsic base and the emitter region, accordingly, a sufficient lateral distance must be provided between the silicide edge and the emitter edge. As a result, the bulk resistance of the extrinsic base is detrimentally increased.
In particular, in the methods as taught in the publications U.S. Pat. Nos. 6,518,111 and 6,239,477, it is endeavored to reduce the base bulk resistance portion due to the non-silicidized region of the extrinsic base. In this regard, a large minimum distance is maintained between the emitter window and the extrinsic base to prevent base-emitter short circuits through laterally growing silicide crystals, the so-called spikes. To reduce the resistance, the region of the extrinsic base, which is situated between the silicide and the intrinsic base, is doped as highly as possible by dopants from a solid dopant source, without detrimentally affecting the properties of the base/emitter PN junction.
U.S. Pat. No. 6,518,111 provides a dielectric diffusion source layer (borosilicate glass) as a dopant source to reduce the layer resistance of the contact region between the intrinsic base and an extrinsic base of polysilicon, whereas U.S. Pat. No. 6,239,477 provides doping between the intrinsic base and a silicidized extrinsic base. US 2004/0014271 provides a selectively epitaxially deposited, highly doped silicon layer as the solid source.